Ribosomal RNA (rRNA) synthesis, the initiation of which is an early major event during the transformation of iris into lens in the newt, was characterized in the TVI cell-line derived from the eastern North American newt Notophthalmus viridescens. Employing the technique of polyacrylamide gel electrophoresis, molecular-weight measurements were made on newt rRNAs using Xenopus laevis and E. coli rRNAs as standards. The molecular weights of N. viridescens 28S and 18S rRNA were found to be 1.4 x 10 to the 6th power and 0.7 x 10 to the 6th power respectively. The precursor to these RNAs had a molecular weight of 3.1 x 10 to the 6th power. Three probable intermediates in the processing of precursor to mature rRNA were also identified. On the basis of the molecular weights of all species of RNA identified, a processing pathway, similar to that of Xenopus, has been suggested. Some unusual features in the kinetics of precursor rRNA labelling and processing suggest the possibility that newt-cell rRNA synthesis may be controlled by the availability of essential amino acids in a manner similar to that observed in mammalian cells. A possible relationship between the availability of essential amino acids, the initiation of rRNA synthesis in the newt iris, and the control of lens regeneration is discussed. BIBLIOGRAPHIC REFERENCE: Reese,D.: Ribosomal RNA in the eastern North-American newt, Notophthalmus viridescens. Differentiation 7: 99-106, 1977.